_Red Milk._ _Bacillus prodigiosus_, in the presence of oxygen, causes a redness, particularly on the surface of milk. It was the work of this bacillus that caused "the bleeding host," which was one of the superstitions of the Middle Ages. _B. lactis erythrogenes_ produces a red colour only in the dark, and in milk that is not strongly acid in reaction. When grown in the light this organism produces a yellow colour. There is a red sarcina (_Sarcina rosea_) which also has the faculty of producing red pigment. One of the yeasts is another example. It must not be forgotten that redness in milk may actually be due to the presence of blood from the udder of the cow. In such a case the blood and milk will be inextricably mixed together, and not in patches or a pellicle. _Blue Milk_ is due to the growth of _Bacillus cyanogenus_. This is an actively motile rod, the presence of which does not materially affect the milk, but causes the milk products to be of poor quality. _Yellow Milk._ _Bacillus synxanthus_ is held responsible for curdling the milk, and then at a later stage, in redissolving the curd, produces a yellow pigment. _Violet and Green Pigments_ in milk are also the work of various bacteria. 2. _Various Unclassified Bacteria_ In milk this is a comparatively small group, for it happens that those bacteria in milk which cannot be classified as fermentative or pathogenic are few. The almost ubiquitous _Bacillus coli communis_ occurs here as elsewhere, and might be grouped with the gaseous fermentative organisms on account of its extraordinary power of producing gas and breaking up the medium (whether agar or cheese) in which it is growing. What its exact rôle is in milk it would be difficult to say. It may act, as it frequently does elsewhere, by _association_ in various fermentations. Some authorities hold that its presence in excessive numbers may cause epidemic diarrhœa in infants. Several years ago a commission was appointed by the _British Medical Journal_ to inquire into the quality of the milk sold in some of the poorer districts of London. Every sample was found to contain _Bacillus coli_, and it was declared that this particular microbe constituted 90 per cent. of all the organisms found in the milk.[57] We record this statement, but accept it with some misgiving. The diagnosis of _B. coli_ four or five years ago was not such a strict matter as to-day. Still, undoubtedly, this particular organism is not uncommonly found in milk, and its source is unclean dairying. In the same investigation _Proteus vulgaris_, _B. fluorescens_, and many liquefying bacteria were frequently found. Their presence in milk means contamination with putrefying matter, surface water, or a foul atmosphere. A number of water bacteria find their way into milk in the practice of adulteration, and foul byres afford ample opportunity for aërial pollution.